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Weather Happenings Friday 9/21

32 Hour Roller Coaster

A decent autumn low pressure system moved in over the past 24 hours.* Pressure is heading up now, and it looks to be sunny tomorrow with normal temps (low 60s) for late September.

Autumn is typically when we start seeing big wind storms. Our max gust so far today, Friday, has been 37 mph at 11:30AM. Winds are beginning to slightly trend down as of 5PM.

Graphic courtesy of Intellicast.
Find it anytime on our Area page using the “Zone” selector at the top.

Summer Below the Bridge

Check out the big spread in temps across the state currently. It’s still in the 80s in parts of Lower Michigan!

Town should escape frost for the most part tonight, but it might be our coldest night since at least 9/9 (low of 44) if not last Spring.

As always, we archive everything we measure on the weather history page. That’s where you can find out what weather is typically like this time of year or what it’s been like in past years.

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* We hit a minimum sea-level pressure of 29.33″ (993mb) at 7AM this morning. Not a shabby number, but we’ve seen lower. 

 

The Deluge Overwhelmed Our Rain Gauge

Manual rain gauge with 11″ capacity

We’ve received 3.11″ of rain at our location just north of the Vet’s Home since midnight. That’s an all-time record for our station.

Unfortunately, our automated rain gauge had some issues overnight and missed 1 full inch of rain. Yikes! Normally, it is pretty accurate. For the last two big rainfalls (on 9/1 and 8/27) it read within a few percent of our reference gauge. I’ve now inspected the tipping bucket, cleaned it out (the cups were pretty dirty), and returned it to service. I’ll keep an eye on it.

For today (9/5), I’ve overridden the automatic gauge with the manual measurement. You’ll see the wrong value in the stats graph at the top of the stats page. The precipitation tables further down the page should be correct, though.

How big was that rain event? It looks like Marquette set a new daily rainfall record for September 5th for the 24 hours ending at 7AM this morning. The official COOP station at the Water Treatment Plant on Lakeshore Dr. reported 3.41″ during that time. The old record was 1.3″ set in 2007. By comparison, for the 24 hours ending at 8AM this morning, we measured 3.29″.

Marquette has seen even greater rainfall in its history. On September 29, 1881, 4.44″ fell!

Altogether since August 27th, we’ve recorded 7.51″ of rain! I don’t know if that’s a record stretch for the city, but it sure knocks my socks off!

We took a small amount of water into our basement. Nothing serious, though. Apparently, we aren’t alone as there are reports around town of basement flooding. Let us know in the comments if you’ve been affected by all this rain we’ve had.

Adjusted Humidity Readings

We are now adding 5% when humidity readings reach 95% since that is the maximum our sensor will read. For many years Davis Instruments’ sensors have struggled to achieve 100% humidity. Unfortunately, the problem seems to worsen with time. When we placed our sensor in service last November 28th, it would reach 98%.

Also, we are adding 1% to 92% (to make 93%) , 2% to 93% (to make 95%), and 4% to 94% to reduce the jump between 91% and 100%. Tests demonstrate these adjustments closely approximate reality.

You may occasionally see other readings besides 93%, 95%, 98%, and 100% due to 5 minute averaging of humidity values.

Raw values are available on the stats page.

Here’s some more info on this problem if you’re interested:

We have a backup weather station for emergencies. It also contains a Davis VP2 station with an SHT31 sensor that is less than 2 years old. It maxes out at 96% humidity, but it also has an average +8% bias below 80% during the warm season (May – Sept). So it has a wet bias in the middle range in addition to a dry bias in the top range. Keep in mind that the maximum error for this sensor is supposed to be 3.5% according to the manufacturer.

We are not the only ones to experience these issues. Many other Davis VP2 weather station owners have reported them (in a thread approaching 1200 posts). Davis says they are “looking into it”, although that was almost 2 months ago now. Some particularly brave souls have attempted to informally wire up older (SHT11/SHT15) and/or unsupported sensors (SHT75) with varying degrees of luck. We’re not keen on performing ad hoc experiments with our primary temperature/humidity sensor. Some combination of Murphy’s Law and common sense would argue that such slapdash “solutions” will inevitably break at the worst time. Indeed our various attempts with other sensors on the test bench have not gone well. Therefore, we await a viable, proven fix preferably from Davis Instruments.

[UPDATE 9/21/18: Most of those enterprising individuals referenced above are now indicating an array of problems with the SHT75 sensor ranging from sudden failure to gross errors. It’s uncertain whether the fault lies with the sensor or the implementation. Either way, at present there appears to be no good workaround to the issues mentioned above with the SHT31.]

[UPDATE 10/23/18]: We recently swapped out our SHT31 sensor suite as part of a preventative maintenance routine. The new humidity sensor arrived with a +6% average (wet) bias below 55F, an average +4% bias above 55F and a maximum reading of 98% (after several hours in saturated air). We have corrected these deficiencies with custom programming.

When Weather Apps Fail at Weather

I wrote about this yesterday on Twitter. Given the limitations of that platform, however, I couldn’t really go deep. Let’s dive in.

Here are two screenshots taken within 60 seconds yesterday afternoon:

 

The 1st image is of a popular weather app recently redesigned for the iPhone. The 2nd image is our universal web app loaded in Safari on an iPhone.*

Continue reading “When Weather Apps Fail at Weather”